The easiest way to use a FunctionCallback is through an anonymous inner class. Override
the done function to specify what the callback should do after the cloud function is
complete. The done function will be run in the UI thread, while the fetch happens in
a background thread. This ensures that the UI does not freeze while the fetch happens.

For example, this sample code calls a cloud function "MyFunction" with
params and calls a different function depending on whether the function succeeded.